External defibrillator capable of delivering patient impedance compensated biphasic waveforms

ABSTRACT

This invention provides an external defibrillator and defibrillation method that automatically compensates for patient-to-patient impedance differences in the delivery of electrotherapeutic pulses for defibrillation and cardioversion. In a preferred embodiment, the defibrillator has an energy source that may be discharged through electrodes on the patient to provide a biphasic voltage or current pulse. In one aspect of the invention, the first and second phase duration and initial first phase amplitude are predetermined values. In a second aspect of the invention, the duration of the first phase of the pulse may be extended if the amplitude of the first phase of the pulse fails to fall to a threshold value by the end of the predetermined first phase duration, as might occur with a high impedance patient. In a third aspect of the invention, the first phase ends when the first phase amplitude drops below a threshold value or when the first phase duration reaches a threshold time value, whichever comes first, as might occur with a low to average impedance patient. This method and apparatus of altering the delivered biphasic pulse thereby compensates for patient impedance differences by changing the nature of the delivered electrotherapeutic pulse, resulting in a smaller, more efficient and less expensive defibrillator.

This application is a continuation of application Ser. No. 08/803,094filed Feb. 20, 1997, now U.S. Pat. No. 5,735,879, which is acontinuation of application Ser. No. 08/103,837 filed Aug. 6, 1993, nowabandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to an electrotherapy method andapparatus for delivering a shock to a patient's heart. In particular,this invention relates to a method and apparatus for using an externaldefibrillator to deliver a biphasic defibrillation shock to a patient'sheart through electrodes attached to the patient.

Defibrillators apply pulses of electricity to a patient's heart toconvert ventricular arrhythmias, such as ventricular fibrillation andventricular tachycardia, to normal heart rhythms through the processesof defibrillation and cardioversion, respectively. There are two mainclassifications of defibrillators: external and implanted. Implantabledefibrillators are surgically implanted in patients who have a highlikelihood of needing electrotherapy in the future. Implanteddefibrillators typically monitor the patient's heart activity andautomatically supply electrotherapeutic pulses directly to the patient'sheart when indicated. Thus, implanted defibrillators permit the patientto function in a somewhat normal fashion away from the watchful eye ofmedical personnel.

External defibrillators send electrical pulses to the patient's heartthrough electrodes applied to the patient's torso. Externaldefibrillators are useful in the emergency room, the operating room,emergency medical vehicles or other situations where there may be anunanticipated need to provide electrotherapy to a patient on shortnotice. The advantage of external defibrillators is that they may beused on a patient as needed, then subsequently moved to be used withanother patient. However, because external defibrillators deliver theirelectrotherapeutic pulses to the patient's heart indirectly (i.e., fromthe surface of the patient's skin rather than directly to the heart),they must operate at higher energies, voltages and/or currents thanimplanted defibrillators. The high energy, voltage and currentrequirements have made current external defibrillators large, heavy andexpensive, particularly due to the large size of the capacitors or otherenergy storage media required by these prior art devices.

The time plot of the current or voltage pulse delivered by adefibrillator shows the defibrillator's characteristic waveform.Waveforms are characterized according to the shape, polarity, durationand number of pulse phases. Most current external defibrillators delivermonophasic current or voltage electrotherapeutic pulses, although somedeliver biphasic sinusoidal pulses. Some prior art implantabledefibrillators, on the other hand, use truncated exponential, biphasicwaveforms. Examples of biphasic implantable defibrillators may be foundin U.S. Pat. No. 4,821,723 to Baker, Jr., et al.; U.S. Pat. No.5,083,562 to de Coriolis et al.; U.S. Pat. No. 4,800,883 to Winstrom;U.S. Pat. No. 4,850,357 to Bach, Jr.; and U.S. Pat. No. 4,953,551 toMehra et al.

Because each implanted defibrillator is dedicated to a single patient,its operating parameters, such as electrical pulse amplitudes and totalenergy delivered, may be effectively titrated to the physiology of thepatient to optimize the defibrillator's effectiveness. Thus, forexample, the initial voltage, first phase duration and total pulseduration may be set when the device is implanted to deliver the desiredamount of energy or to achieve that desired start and end voltagedifferential (i.e, a constant tilt).

In contrast, because external defibrillator electrodes are not in directcontact with the patient's heart, and because external defibrillatorsmust be able to be used on a variety of patients having a variety ofphysiological differences, external defibrillators must operateaccording to pulse amplitude and duration parameters that will beeffective in most patients, no matter what the patient's physiology. Forexample, the impedance presented by the tissue between externaldefibrillator electrodes and the patient's heart varies from patient topatient, thereby varying the intensity and waveform shape of the shockactually delivered to the patient's heart for a given initial pulseamplitude and duration. Pulse amplitudes and durations effective totreat low impedance patients do not necessarily deliver effective andenergy efficient treatments to high impedance patients.

Prior art external defibrillators have not fully addressed the patientvariability problem. One prior art approach to this problem was toprovide the external defibrillator with multiple energy settings thatcould be selected by the user. A common protocol for using such adefibrillator was to attempt defibrillation at an initial energy settingsuitable for defibrillating a patient of average impedance, then raisethe energy setting for subsequent defibrillation attempts in the eventthat the initial setting failed. The repeated defibrillation attemptsrequire additional energy and add to patient risk. What is needed,therefore, is an external defibrillation method and apparatus thatmaximizes energy efficiency (to minimize the size of the required energystorage medium) and maximizes therapeutic efficacy across an entirepopulation of patients.

SUMMARY OF THE INVENTION

This invention provides an external defibrillator and defibrillationmethod that automatically compensates for patient-to-patient impedancedifferences in the delivery of electrotherapeutic pulses fordefibrillation and cardioversion. In a preferred embodiment, thedefibrillator has an energy source that may be discharged throughelectrodes on the patient to provide a biphasic voltage or currentpulse. In one aspect of the invention, the first and second phaseduration and initial first phase amplitude are predetermined values. Ina second aspect of the invention, the duration of the first phase of thepulse may be extended if the amplitude of the first phase of the pulsefails to fall to a threshold value by the end of the predetermined firstphase duration, as might occur with a high impedance patient. In a thirdaspect of the invention, the first phase ends when the first phaseamplitude drops below a threshold value or when the first phase durationreaches a threshold time value, whichever comes first, as might occurwith a low to average impedance patient. This method and apparatus ofaltering the delivered biphasic pulse thereby compensates for patientimpedance differences by changing the nature of the deliveredelectrotherapeutic pulse, resulting in a smaller, more efficient andless expensive defibrillator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a low-tilt biphasicelectrotherapeutic waveform according to a first aspect of thisinvention.

FIG. 2 is a schematic representation of a high-tilt biphasicelectrotherapeutic waveform according to the first aspect of thisinvention.

FIG. 3 is a flow chart demonstrating part of an electrotherapy methodaccording to a second aspect of this invention.

FIG. 4 is a schematic representation of a biphasic waveform deliveredaccording to the second aspect of this invention.

FIG. 5 is a schematic representation of a biphasic waveform deliveredaccording to the second aspect of this invention.

FIG. 6 is a flow chart demonstrating part of an electrotherapy methodaccording to a third aspect of this invention.

FIG. 7 is a schematic representation of a biphasic waveform deliveredaccording to the third aspect of this invention.

FIG. 8 is a schematic representation of a biphasic waveform deliveredaccording to the third aspect of this invention.

FIG. 9 is a flow chart demonstrating part of an electrotherapy methodaccording to a combination of the second and third aspects of thisinvention.

FIG. 10 is a block diagram of a defibrillator system according to apreferred embodiment of this invention.

FIG. 11 is a schematic circuit diagram of a defibrillator systemaccording to a preferred embodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1 and 2 illustrate the patient-to-patient differences that anexternal defibrillator design must take into account. These figures areschematic representations of truncated exponential biphasic waveformsdelivered to two different patients from an external defibrillatoraccording to the electrotherapy method of this invention fordefibrillation or cardioversion. In these drawings, the vertical axis isvoltage, and the horizontal axis is time. The principles discussed hereare applicable to waveforms described in terms of current versus time aswell, however.

The waveform shown in FIG. 1 is called a low-tilt waveform, and thewaveform shown in FIG. 2 is called a high-tilt waveform, where tilt H isdefined as a percent as follows: ##EQU1## As shown in FIGS. 1 and 2, Ais the initial first phase voltage and D is the second phase terminalvoltage. The first phase terminal voltage B results from the exponentialdecay over time of the initial voltage A through the patient, and thesecond phase terminal voltage D results from the exponential decay ofthe second phase initial voltage C in the same manner. The startingvoltages and first and second phase durations of the FIG. 1 and FIG. 2waveforms are the same; the differences in end voltages B and D reflectdifferences in patient impedance.

Prior art disclosures of the use of truncated exponential biphasicwaveforms in implantable defibrillators have provided little guidancefor the design of an external defibrillator that will achieve acceptabledefibrillation or cardioversion rates across a wide population ofpatients. The defibrillator operating voltages and energy deliveryrequirements affect the size, cost, weight and availability ofcomponents. In particular, operating voltage requirements affect thechoice of switch and capacitor technologies. Total energy deliveryrequirements affect defibrillator battery and capacitor choices.

We have determined that, for a given patient, externally-appliedtruncated exponential biphasic waveforms defibrillate at lower voltagesand at lower total delivered energies than externally-applied monophasicwaveforms. In addition, we have determined that there is a complexrelationship between total pulse duration, first to second phaseduration ratio, initial voltage, total energy and total tilt.

Up to a point, the more energy delivered to a patient in anelectrotherapeutic pulse, the more likely the defibrillation attemptwill succeed. Low-tilt biphasic waveforms achieve effectivedefibrillation rates with less delivered energy than high-tiltwaveforms. However, low-tilt waveforms are energy inefficient, sincemuch of the stored energy is not delivered to the patient. On the otherhand, defibrillators delivering high-tilt biphasic waveforms delivermore of the stored energy to the patient than defibrillators deliveringlow-tilt waveforms while maintaining high efficacy up to a certaincritical tilt value. Thus, for a given capacitor, a given initialvoltage and fixed phase durations, high impedance patients receive awaveform with less total energy and lower peak currents but betterconversion properties per unit of energy delivered, and low impedancepatients receive a waveform with more delivered energy and higher peakcurrents. There appears to be an optimum tilt range in which high andlow impedance patients will receive effective and efficient therapy. Anoptimum capacitor charged to a predetermined voltage can be chosen todeliver an effective and efficient waveform across a population ofpatients having a variety of physiological differences.

This invention is a defibrillator and defibrillation method that takesadvantage of this relationship between waveform tilt and total energydelivered in high and low impedance patients. In one aspect of theinvention, the defibrillator operates in an open loop, i.e., without anyfeedback regarding patient impedance parameters and with preset pulsephase durations. The preset parameters of the waveforms shown in FIGS. 1and 2 are therefore the initial voltage A of the first phase of thepulse, the duration E of the first phase, the interphase duration G, andthe duration F of the second phase. The terminal voltage B of the firstphase, the initial voltage C of the second phase, and the terminalvoltage D of the second phase are dependent upon the physiologicalparameters of the patient and the physical connection between theelectrodes and the patient.

For example, if the patient impedance (i.e., the total impedance betweenthe two electrodes) is high, the amount of voltage drop (exponentialdecay) from the initial voltage A to the terminal voltage B during timeE will be lower (FIG. 1) than if the patient impedance is low (FIG. 2).The same is true for the initial and terminal voltages of the secondphase during time F. The values of A, E, G and F are set to optimizedefibrillation and/or cardioversion efficacy across a population ofpatients. Thus, high impedance patients receive a low-tilt waveform thatis more effective per unit of delivered energy, and low impedancepatients receive a high-tilt waveform that delivers more of the storedenergy and is therefore more energy efficient.

Another feature of biphasic waveforms is that waveforms with relativelylonger first phases have better conversion properties than waveformswith equal or shorter first phases, provided the total duration exceedsa critical minimum. Therefore, in the case of high impedance patients,it may be desirable to extend the first phase of the biphasic waveform(while the second phase duration is kept constant) to increase theoverall efficacy of the electrotherapy by delivering a more efficaciouswaveform and to increase the total amount of energy delivered. FIGS. 3-5demonstrate a defibrillation method according to this second aspect ofthe invention in which information related to patient impedance is fedback to the defibrillator to change the parameters of the deliveredelectrotherapeutic pulse.

FIG. 3 is a flow chart showing the method steps following the decision(by an operator or by the defibrillator itself) to apply anelectrotherapeutic shock to the patient through electrodes attached tothe patient and charging of the energy source, e.g., the defibrillator'scapacitor or capacitor bank, to the initial first phase voltage A. Block10 represents initiation of the first phase of the pulse in a firstpolarity. Discharge may be initiated manually by the user orautomatically in response to patient heart activity measurements (e.g.,ECG signals) received by the defibrillator through the electrodes andanalyzed by the defibrillator controller in a manner known in the art.

Discharge of the first phase continues for at least a threshold timet_(THRESH), as shown by block 12 of FIG. 3. If, at the end of timet_(THRESH), the voltage measured across the energy source has notdropped below the minimum first phase terminal voltage thresholdV_(THRESH), first phase discharge continues, as shown in block 14 ofFIG. 3. For high impedance patients, this situation results in anextension of the first phase duration beyond t_(THRESH), as shown inFIG. 4, until the measured voltage drops below the threshold V_(THRESH).Discharge then ends to complete the first phase, as represented by block16 of FIG. 3. If, on the other hand, the patient has low impedance, thevoltage will have dropped below V_(THRESH) when the time threshold isreached, resulting in a waveform like the one shown in FIG. 5.

At the end of the first phase, and after a predetermined interim periodG, the polarity of the energy source connection to the electrodes isswitched, as represented by blocks 18 and 20 of FIG. 3. Discharge of thesecond phase of the biphasic pulse then commences and continues for apredetermined second phase duration F, as represented by block 26 ofFIG. 3, then ceases. This compensating electrotherapy method ensuresthat the energy is delivered by the defibrillator in the mostefficacious manner by providing for a minimum waveform tilt and byextending the first phase duration to meet the requirements of aparticular patient.

Because this method increases the waveform tilt for high impedancepatients and delivers more of the energy from the energy source than amethod without compensation, the defibrillator's energy source can besmaller than in prior art external defibrillators, thereby minimizingdefibrillator size, weight and expense. It should be noted that thewaveforms shown in FIGS. 4 and 5 could be expressed in terms of currentversus time using a predetermined current threshold value withoutdeparting from the scope of the invention.

FIGS. 6-8 illustrate a third aspect of this invention that prevents thedelivered waveform from exceeding a maximum tilt (i.e., maximumdelivered energy) in low impedance patients. As shown by blocks 52 and54 in FIG. 6, the first phase discharge stops either at the end of apredetermined time t_(THRESH) or when the first phase voltage dropsbelow V'_(THRESH). The second phase begins after an interim period G andcontinues for a preset period F as in the second aspect of theinvention. Thus, in high impedance patients, the first phase ends attime t_(THRESH), even if the voltage has not yet fallen belowV'_(THRESH), as shown in FIG. 7. In low impedance patients, on the otherhand, the first phase of the delivered waveform could be shorter induration than the time t_(THRESH), as shown in FIG. 8.

Once again, the waveforms shown in FIGS. 7 and 8 could be expressed interms of current versus time using a predetermined current thresholdvalue without departing from the scope of the invention.

FIG. 9 is a flow chart illustrating a combination of the defibrillationmethods illustrated in FIGS. 3 and 6. In this combination method, thefirst phase of the biphasic waveform will end if the voltage reaches afirst voltage threshold V'_(THRESH) prior to the first phase durationthreshold t_(THRESH), as shown by blocks 91 and 92. This defibrillatordecision path delivers a waveform like that shown in FIG. 8 for lowimpedance patients. For high impedance patients, on the other hand, ifat the expiration of t_(THRESH) the voltage has not fallen belowV'_(THRESH), the duration of the first phase is extended beyondt_(THRESH) until the voltage measured across the electrodes reaches asecond voltage threshold V_(THRESH), as shown in decision blocks 91 and93. This defibrillator method path will deliver a waveform like thatshown in FIG. 4.

In alternative embodiments of this invention, the second phase pulsecould be a function of the first phase voltage, current or time insteadof having a fixed time duration. In addition, any of the aboveembodiments could provide for alternating initial polarities insuccessive monophasic or biphasic pulses. In other words, if in thefirst biphasic waveform delivered by the system the first phase is apositive voltage or current pulse followed by a second phase negativevoltage or current pulse, the second biphasic waveform delivered by thesystem would be a negative first phase voltage or current pulse followedby a positive second phase voltage or current pulse. This arrangementwould minimize electrode polarization, i.e., build-up of charge on theelectrodes.

For each defibrillator method discussed above, the initial first phasevoltage A may be the same for all patients or it may be selectedautomatically or by the defibrillator user. For example, thedefibrillator may have a selection of initial voltage settings, one foran infant, a second for an adult, and a third for use in open heartsurgery.

FIG. 10 is a schematic block diagram of a defibrillator system accordingto a preferred embodiment of this invention. The defibrillator system 30comprises an energy source 32 to provide the voltage or current pulsesdescribed above. In one preferred embodiment, energy source 32 is asingle capacitor or a capacitor bank arranged to act as a singlecapacitor. A connecting mechanism 34 selectively connects anddisconnects energy source 32 to and from a pair of electrodes 36electrically attached to a patient, represented here as a resistive load37. The connections between the electrodes and the energy source may bein either of two polarities with respect to positive and negativeterminals on the energy source.

The defibrillator system is controlled by a controller 38. Specifically,controller 38 operates the connecting mechanism 34 to connect energysource 32 with electrodes 36 in one of the two polarities or todisconnect energy source 32 from electrodes 36. Controller 38 receivestiming information from a timer 40, and timer 40 receives electricalinformation from electrical sensor 42 connected across energy source 32.In some preferred embodiments, sensor 42 is a voltage sensor; in otherpreferred embodiments, sensor 42 is a current sensor.

FIG. 11 is a schematic circuit diagram illustrating a device accordingto the preferred embodiments discussed above. Defibrillator controller70 activates a high voltage power supply 72 to charge storage capacitor74 via diode 76 to a predetermined voltage. During this period, switchesSW1, SW2, SW3 and SW4 are turned off so that no voltage is applied tothe patient (represented here as resistor 78) connected betweenelectrodes 80 and 82. SW5 is turned on during this time.

After charging the capacitor, controller 70 de-activates supply 72 andactivates biphase switch timer 84. Timer 84 initiates discharge of thefirst phase of the biphasic waveform through the patient in a firstpolarity by simultaneously turning on switches SW1 and SW4 via controlsignals T1 and T4, while switch SW5 remains on to deliver the initialvoltage A through electrodes 80 and 82 to the patient 78.

Depending on the operating mode, delivery of the first phase of thebiphasic pulse may be terminated by the timer 84 after the end of apredetermined period or when the voltage across the electrodes hasdropped below a predetermined value as measured by comparator 86. Timer84 terminates pulse delivery by turning off switch SW5 via controlsignal T5, followed by turning off switches SW1 and SW4. The voltageacross electrodes 80 and 82 then returns to zero.

During the interim period G, SW5 is turned on to prepare for the secondphase. After the end of interim period G, timer 84 initiates delivery ofthe second phase by simultaneously turning on switches SW2 and SW3 viacontrol signals T2 and T3 while switch SW5 remains on. Thisconfiguration applies voltage from the capacitor to the electrodes at aninitial second phase voltage C and in a polarity opposite to the firstpolarity. Timer 84 terminates delivery of the second phase by turningoff switch SW5 via control signal T5, followed by turning off switchesSW2 and SW3. The second phase may be terminated at the end of apredetermined period or when the voltage measured by comparator 86 dropsbelow a second phase termination voltage threshold.

In a preferred embodiment, switch SW5 is an insulated gate bipolartransistor (IGBT) and switches SW1-SW4 are silicon-controlled rectifiers(SCRs). The SCRs are avalanche-type switches which can be turned on to aconductive state by the application of a control signal, but cannot beturned off until the current through the switch falls to zero or nearzero. Thus, the five switches can be configured so that any of theswitches SW1-SW4 will close when SW5 is closed and will reopen only uponapplication of a specific control signal to SW5.

This design has the further advantage that switch SW5 does not need towithstand the maximum capacitor voltage. The maximum voltage that willbe applied across switch SW5 will occur when the first phase isterminated by turning SW5 off, at which time the capacitor voltage hasdecayed to some fraction of its initial value.

Other switches and switch configurations may be used, of course withoutdeparting from the scope of the invention. In addition, thedefibrillator configurations of FIGS. 10 and 11 may be used to deliverelectric pulses of any polarity, amplitude, and duration singly and inany combination.

While the invention has been discussed with reference to externaldefibrillators, one or more aspects of the invention would be applicableto implantable defibrillators as well. Other modifications will beapparent to those skilled in the art.

What is claimed is:
 1. An external defibrillator comprising:an energysource; first and second electrodes in electrical communication with theexterior of a patient; a controller controlling application ofelectrical energy from the energy source to the electrodes, thecontroller comprising, a timer for generating a timing signalcorresponding to the application of electrical energy from the energysource to the electrodes, and means for selectively connecting theenergy source to the electrodes in a first polarity and a secondpolarity.
 2. The apparatus of claim 1 wherein the controller furthercomprises means for measuring an electrical unit of the energy sourceapplied across the electrodes and for generating a signal correspondingto the measured electrical unit.
 3. The apparatus of claim 2 wherein theenergy source is a voltage source and the measured electrical unit isvoltage.
 4. The apparatus of claim 3 wherein the voltage sourcecomprises a capacitor.
 5. The apparatus of claim 2 wherein the energysource is a current source and the measured electrical unit is current.6. The apparatus of claim 2 wherein the controller further comprisesthreshold means for determining whether the measured electrical unitsignal is less than a predetermined threshold value.
 7. The apparatus ofclaim 6 wherein the means for selectively connecting comprises a firstswitch for connecting a first terminal of the energy source with thefirst electrode; a second switch for connecting the first terminal ofthe energy source with the second electrode; a third switch forconnecting the first electrode with a junction; a fourth switch forconnecting the second electrode with the junction; and a fifth switchfor connecting the junction with the second terminal of the energysource.
 8. An external defibrillator comprising:an energy source; firstand second electrodes adapted to make contact with a patient; aplurality of electronic switches; a controller controlling applicationof electrical energy from the energy source to the electrodes throughthe electronic switches in a truncated exponential multiphasic waveform.9. The external defibrillator of claim 8 wherein the controller furthercomprises a timer.
 10. The external defibrillator of claim 8 furthercomprising means for limiting voltage applied across one of theplurality of electronic switches.
 11. The external defibrillator ofclaim 10, the plurality of electronic switches comprising:a first switchcoupled between a first terminal of the energy source and the firstelectrode; a second switch coupled between the first terminal of theenergy source and the second electrode; a third switch coupled betweenthe first electrode and a second terminal of the energy source; a fourthswitch coupled between the second electrode and the second terminal ofthe energy source; andthe means for limiting voltage comprising a fifthswitch interposed between the second terminal and the third and fourthswitches of the energy source; wherein any of the first, second, thirdand fourth switches may close only when the fifth switch is closed andmay re-open only when the fifth switch is open.
 12. The externaldefibrillator of claim 8 wherein the energy source consists of a singlecapacitor.